1. Field of the Invention
The invention is directed to a technique for aligning laser beam optics including transport system optics and laser cavity optics.
2. Description of the Prior Art
(a) Laser Transport System Optics
In the use of lasers it is frequently necessary to transport the laser beams over large distances and use several mirrors to deflect the beam around obstacles, through wall openings and ultimately focus the laser beam on a work piece. Such beam transport systems are frequently encountered in high power laser systems where several work stations may be used. In the alignment of such beam transport systems, it is common practice to use a visible alignment laser beam, such as that emitted by a HeNe laser, as a tracking beam for visual alignment of the system. This is especialy important in certain applications for two reasons. First, the laser beams of some laser systems, notably CO.sub.2 lasers, YAG lasers and excimer lasers, are not visible and their propagation through a complex optical system cannot be easily observed. Second, these lasers frequently operate at such power levels that it is unsafe to turn them on without being sure where the beams will impact. An example of how an alignment laser system can be used to align a high power laser optical system can be explained with the use of FIG. 1. This technique combines elements of common practice and some procedures disclosed in U.S. Pat. No. 4,466,739.
In order to simplify the following descriptions, reference to mirrors, targets and detectors will sometimes be made alone (e.g., Mo, M', M", T1, T2 . . . , Da, Db . . . , etc.) without referring to function (e.g., observation mirror, transparent mirror, etc.). Also, unless necessary for directional orientation, numbered or lettered designations of mirrors or detectors, etc. will not be used.
Prior to aligning the optical system the alignment laser 10 (sometimes referred to as HeNe laser) must be adjusted so that its visible beam B, illustrated by the long/short dashed line, tracks the path followed by the beam P of a high power laser (not shown), illustrated in solid lines of a high power laser (not shown). In FIG. 1, apertured observation mirror Mo directs the image to a viewing system (shown schematically as an eye E) for observation of reflected signals from the optical system. The alignment beam passes through hole H in Mo and is reflected from the surface of first deflecting mirror M'. (The mounting of M' is fabricated so that the mirror can be lifted out of position and replaced again and maintain the original orientation.) The alignment laser beam B is then reflected off the second deflecting mirror M" which is temporarily located in the position shown between first deflecting mirror M' and all downstream laser transport mirrors M1-M5. To co-align the alignment beam B with the high power laser beam P, one first removes M' and directs the high power laser beam P against the second deflecting mirror M" along the straight auxiliary path AUX (dotted lines) where it impacts targets that can be located at different points along the path. The high power laser (not shown) is turned off and M' is reinserted, thereby causing the alignment laser beam B to traverse the auxiliary path AUX. The orientation of the alignment laser 10 is then adjusted until its beam B strikes the centers of movable targets previously irradiated with the high power laser beam P. When this condition is achieved, the alignment laser beam B reflected from M' is co-linear with the center of the high power laser beam P, and mirror M" can be removed.
In aligning the optical system exemplified in FIG. 1, the high power laser beam P is turned off and the mirror M' is kinematically located in position. With the alignment laser 10 turned on, the transport mirror M1 is positioned so that the beam B strikes its center section. The target T2, indicated in dashed lines, is then inserted into the beam path in front of mirror M2. This target T2 should be made of a material that will scatter the beam B. This scattered radiation can be observed by looking directly at T2. Alternately, a part of the radiation will scatter back through the optical system, i.e., reflect off M1, M', and Mo and into viewing system E. The mirror M1 is then adjusted until the beam B strikes T2 at a previously determined position that corresponds to striking mirror M2 at the center position. The target T2 is then removed, either mechanically or by hand (see the double headed arrow), and target T3 is positioned in place in front of M3. The mirror M2 is adjusted until the beam B strikes the proper spot on T3. This procedure is repeated for all transport mirrors, M1- M5, etc., until the beam B is finally projected onto the focal spot F where the high power laser beam P will ultimately be focused for the proposed application. At this point the preliminary alignment of the laser beam transport system is completed. The final check of the alignment is made by removing the first deflecting mirror M' and turning on the high power laser (not shown).
To use this alignment technique, it is necessary to insert targets T2, T3 . . . into the optical path and observe where the beam B impacts each target. As noted, the insertion of the targets can be done either mechanically or by hand. In the former case, one must have a mechanical means to accurately position each target at its proper location and then be able to remove it and, as necessary, replace it again accurately time after time. In the latter case, one must have access to all mirror locations to position the targets. Also, with this alignment technique, one must observe where the beam B strikes each target either directly or by some remote method such as described above. (Alternately, techniques involving video monitors or quadrant detectors could also be used to monitor the positions where the HeNe laser beam strikes the targets). In any event, such systems are complex and sometimes difficult to implement efficiently.
(b) Laser Cavity Allignment
It is customary to use unstable resonator cavity optics in high power pulsed/CW laser systems. Such laser optics 12, shown schematically in FIG. 9, tend to extract the most energy from the gain medium and, in addition, produce beams that progagate well. The unstable resonator laser cavity optics usually consist of a concave mirror M2C and a smaller convex mirror, M1C located on the axis A--A' of laser gain medium G. The laser beam P emitted by such a system (shown in dotted lines) has an annular cross-section. Sometimes the mirror M1C is located off the center position. In this case the cross-section of the laser beam is "U" shaped or "L" shaped.
The conventional alignment of unstable resonator laser cavity optics, while quite simple in principle, frequently proves to be a challenge because it requires an invasion of the laser cavity or the laser optical system. In large high-power lasers it is not always simple, or in fact possible, to locate optical components within the laser cavity.
Two general techniques have been used to align unstable resonator laser cavity optics. The most straight forward technique for alignment is shown schematically in FIG. 10. An auxiliary optical system consisting of alignment laser 10 (e.g. a HeNe laser), a thin semitransparent pellicle SP, and a small flat mirror M' are located in the laser cavity (shown with gain medium removed). The mirror M' is oriented so that the alignment laser beam B (arrows) striking it is reflected back along the same path. The thin pellicle SP, normally 50% transmitting and 50% reflecting, located in the path X--X' of the alignment laser 10 reflects the alignment laser beam along the desired optical path A--A' of the unstable resonator laser cavity. Once this auxiliary optical system has been installed and adjusted, the alignment of the cavity mirrors M1C and M2C consists simply of adjusting their elevation and azimuth controls until the auxiliary laser beams striking their surfaces are reflected back on themselves. (It has been assumed that M1C and M2C have been physically positioned so that their centers are aligned on the A--A' axis). Once the mirrors M1C and M2C are aligned, the auxiliary optical components must either be removed or positioned out of the way so that they don't interfere with the operation of the laser. The latter option is not always possible.
A second technique for aligning unstable laser cavity optics that does not require the positioning of optical components within the cavity is shown in FIG. 11. In this case a target T is set up perpendicular to the cavity axis A--A' a short distance from M1C. An alignment laser beam B is projected through a small hole h in the target T. This alignment laser beam B, which is oriented parallel to the axis A--A', passes near the edge of M1C and is initially reflected from M2C. The alignment laser beam then reflects many times from the surfaces of M1C and M2C and finally strikes the target T as shown by the dark annular pattern Pa. Diffraction and multiple reflections from M1C and M2C cause the alignment laser beam B to spread over the large area shown. The alignment of M1C and M2C to the axis A--A' is achieved by adjusting their elevation and azimuth controls to achieve a symmetrical and uniform pattern Pa of the alignment laser beam B on the target T. It is necessary to adjust both mirrors M1C and M2C together to achieve this alignment. After the system is aligned, the alignment laser and target must be removed to permit normal operation of the laser system.
The techniques for aligning unstable resonator laser cavity optics described above are typical of the alignment methods now being used. They require an "invasion" of the laser cavity and/or laser optical system and rely on visual observation of the alignment laser beams. These constraints are often undesirable.